Getting the right SEM resolution can optimize your ROI

The scanning electron microscope (SEM) is a critical tool in a broad range of research fields. Within the world of material science, SEMs have emerged as powerful tools for the characterization and development of a plethora of materials. SEM resolution remains a key decision point for many lab managers.

SEM resolution can be anywhere between less than one nanometer and up to tens of nanometers. Our Verios SEM is capable of a resolution of 0.6 nanometers at 30 kV, for example. However, increasing resolutions also increases the cost and complexity of your SEM. So, what is the best option for your material science lab?

SEM resolution explained

The word resolution simply refers to the smallest observable feature in an image. For the human eye, that is about 0.2 mm. SEM resolution is typically between 0.5 and 4 nanometers.

Let us step back and explain how an SEM works, before we dive into the topic of SEM microscope resolution. A scanning electron microscope scans a focused beam of electrons over a sample surface to create a magnified image. The electrons in the beam interact with the sample, producing several signals. These signals allow material science researchers to obtain information about the material composition and surface topography.

While SEMs cannot provide atomic resolution, typical floor model SEMs can achieve resolutions of the order of 1 to 20 nanometers – some SEMs are even capable of sub-nanometer resolutions.

A desktop SEM can typically achieve resolutions of tens of nanometers. However, the Phenom Pharos SEM features a field emission gun source and is capable of resolutions below three nanometers.

Thanks to this nanometer-scale scanning electron microscope resolution, SEMs have opened a whole new world to researchers. A virus, for example, is between 60 and 140 nanometers. A DNA strand has a radius of one nanometer. A glucose molecule is about 0.9 nanometers in size. You can resolve all of these with an SEM.

Within material science research, you can examine metals, alloys, polymers, ceramics, electronics, particles, and organic materials with ease and down to nanometer feature sizes, thanks to the high resolution of an SEM.

Is there an SEM resolution limit?

The maximum SEM resolution depends on multiple factors, including the size of the incident electron beam and the interaction volume of the beam with the sample.

When it comes to the electron spot size, this spot size can be reduced by introducing an aperture into the beam path. You can also reduce the probe size using the condenser lens. However, this also reduces the beam current. So, as you reduce the probe size you will eventually reach a point where imaging is impossible.

For a typical SEM operating at 20 kV, the minimum usable probe size is of the order of 1-3 nanometers.

Higher accelerating voltages can also provide higher resolution, but the damage to the sample also increases. So, a low accelerating voltage – of several kV - is favorable for more delicate samples. But you can increase the resolution by increasing the accelerating voltage.

The choice of electron source also affects resolution. There are two categories of electron sources: thermionic emission (TEM) sources and field emission (FE) sources.

A TEM source is capable of a resolution of 25 nanometers at 1kV. A FE source can achieve around three nanometers. When we move up to 30 kV, a TEM source is capable of around three nanometers and a FE source can achieve around one nanometer resolution.

Also, images obtained with backscattered electrons have a lower resolution than those obtained with secondary electrons, because they originate from deeper within the sample.

To summarize, there are different factors to consider when evaluating the SEM resolution. There are many microscope limitations beyond the resolution to consider as well. Primarily, your samples and applications drive your SEM requirements.

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